Hydrogen sulfide and alkyl mercaptans are a major source of pollution of air streams since they are liberated as waste by-products in a number of chemical processes, such as sulfate or kraft paper pulp manufacture, viscose manufacture, sewage treatment, sulfurretting fatty oils, and producing organic sulfur compounds, as well as petroleum refining and in the production of natural gas and of combustible gases from coal, such as in the coking operations. These sulfur compounds are also present in geothermal steam used in power generating plants.
The use of an aqueous chelated metal catalyst solution for removing hydrogen sulfide from a gas stream is well known in the art. However, those processes relied on amino and polyaminopolyacetic acid chelating ligands such as, nitrilotriacetic acid, ethylenediaminetetraacetic acid, N-hydroxyethyl ethylenediamine triacetic acid, and diethylenetriamine pentaacetic acid and alkali metal salts thereof. In those prior art processes a chelated metal catalyst solution is contacted with hydrogen sulfide-containing gas, known as "sour gas", to effect oxidation of the hydrogen sulfide to elemental sulfur and concomitant reduction of the metal to a lower oxidation state. The catalyst solution is then regenerated for reuse by contacting it with an oxygen-containing gas to oxidize the metal to a higher oxidation state. The elemental sulfur is continuously removed from the process as a solid product with high purity. Illustrative of these oxidation-reduction processes is the description contained in U.S. Pat. No. 4,622,212 (McManus et al.) and the references cited therein.
Ligands, also known as complexing agents, can be any molecule, atom, or ion that is attached to the central atom of a coordination compound. For example, the ammonia molecule in [Co(NH.sub.3).sub.6 ].sup.3+ and the chlorine atoms in PtCl.sub.6 are ligands. Commercially, ligands are used to modify the properties of metals or metal ions. Ligands interact differently with different metal ions thus producing surprising and unexpected properties for the resulting coordination compounds. Ligands with two or more donor atoms coordinated to the same acceptor atom are known as chelating ligands.
The combination of chelating ligands with metals, such as, iron, copper, cobalt, nickel or manganese results in the formation of metal chelates useful as catalysts in the oxidation-reduction processes to convert H.sub.2 S to solid sulfur.
While the processes, and their associated catalyst compositions, described in the art have achieved sufficient efficiency and stability for commercial utilization, a need still exists for alternate and significantly more stable catalyst systems. This invention addresses this need and in particular provides catalyst solutions having more stability against in-process oxidative degradation of the active metal chelate, significantly reduced precipitation as metal sulfide even when substoichiometric iron to sulfur mole ratios are present in the absorber section of the process, and improved stability of the metal chelate against hydrolysis in aqueous solutions having pH values up to 9.0. These and other advantages will become evident from the following more detailed description of the invention.